Pick a calculation mode, set the circuit parameters, and click Calculate. Core formula: VD = k × I × L × Z ÷ 1000 ÷ N (k = 2 for DC/single-phase, √3 for three-phase).
Percent voltage drop for your circuit across neighboring conductor sizes. Your selected (or recommended) size is highlighted in gold; the dashed line marks the NEC 3% branch-circuit recommendation. One size up or down often makes the compliance difference.
Results are engineering estimates based on published conductor characteristics at the selected operating temperature. Actual voltage drop varies with installation conditions. Verify with a licensed electrician before any electrical work.
Typical copper circuits at 75°C in PVC/aluminum conduit, computed with this calculator's NEC-based method — for quick lookup without re-entering numbers.
| Circuit | Wire (Cu) | Approx. Drop | Within NEC 3%? |
|---|---|---|---|
| 120 V · 15 A · 50 ft · 1Ø | 14 AWG | ≈ 3.9% | No — use 12 AWG (≈ 2.5%) |
| 120 V · 20 A · 50 ft · 1Ø | 12 AWG | ≈ 3.3% | Borderline — 10 AWG brings it to ≈ 2% |
| 240 V · 30 A · 100 ft · 1Ø | 10 AWG | ≈ 3.0% | At the limit — 8 AWG recommended |
| 240 V · 50 A · 150 ft · 1Ø | 6 AWG | ≈ 3.1% | Borderline — 4 AWG brings it to ≈ 2% |
| 208 V · 60 A · 200 ft · 3Ø | 4 AWG | ≈ 3.1% | Borderline — 3 AWG clears 3% |
| 480 V · 100 A · 300 ft · 3Ø | 1 AWG | ≈ 1.8% | Yes |
| 12 V DC · 20 A · 25 ft | 10 AWG | ≈ 10.3% | No — low-voltage DC needs large wire; 4 AWG ≈ 2.6% |
Values are rounded estimates for comparison. Run your exact circuit in the calculator above and verify with a licensed electrician.
Three real-world scenarios, worked step by step. Click "Load This Example" to run it in the calculator above.
What voltage drop is, why the NEC cares about it, and what every selector on this calculator actually changes in the math.
What Voltage Drop Is
Every conductor has resistance, so some of the supply voltage is consumed pushing current through the wire itself instead of powering the load. The longer the run, the smaller the conductor, and the higher the current, the more voltage is lost. The symptoms of excessive drop are familiar on any job site: dim or flickering lights at the end of a long run, heaters that underperform, and motors that run hot and fail early because low voltage forces them to draw more current.
The NEC 3% / 5% Recommendations
Informational Notes to NEC 210.19 and 215.2 recommend limiting voltage drop to 3% on a branch circuit and 5% combined across the feeder and branch circuit. These are design recommendations rather than enforceable requirements in most jurisdictions — but they are the targets inspectors, engineers, and manufacturers expect, and some applications (such as sensitive electronics and fire pumps) carry stricter requirements. This calculator's verdict banner grades your circuit against both thresholds.
60 / 75 / 90°C Conductor Temperature
Conductor resistance rises with temperature — roughly 0.3% per °C for copper and aluminum — so the same wire drops more voltage when running hot. The three selectable ratings correspond to the standard NEC insulation temperature columns. 75°C is the everyday industry assumption; 60°C models a lightly loaded conductor; 90°C models a fully loaded conductor with 90°C-rated insulation. Note that NEC 110.14(C) usually limits terminations to the 60°C or 75°C rating regardless of the wire's insulation, which is one of several reasons results at any setting are estimates that a licensed electrician must verify.
PVC / Aluminum vs. Steel Conduit (AC only)
Alternating current in a conductor creates a changing magnetic field, and the raceway around it affects the resulting reactance. Steel conduit is magnetic and raises the effective reactance compared with PVC or aluminum raceway — NEC Chapter 9 Table 9 publishes separate columns for the two cases. On small branch-circuit conductors the difference is minor because resistance dominates; on large feeders, reactance is a substantial share of total impedance and the conduit selection visibly changes the result. DC circuits have no reactance, so the selector hides in DC mode.
NEC (AWG / kcmil) vs. IEC (mm²) Mode
NEC mode covers the North American size ladder from 14 AWG through 1000 kcmil using resistance and reactance characteristics based on NEC Chapter 9 Tables 8 and 9. IEC mode covers the international metric ladder from 1.5 mm² to 630 mm², with resistance computed from standard IEC 60228 copper and aluminum resistivity and corrected to your selected operating temperature. Switching standards also switches the default length unit between feet and meters — either unit works in either mode.
Loads in kW, kVA, and Horsepower
When you enter a load in kW or hp, the calculator first converts it to current using the supply voltage and your power factor (I = P ÷ (V × PF) single-phase, with a √3 factor for three-phase). kVA entries divide by voltage alone, since kVA already represents apparent power. For horsepower, 1 hp = 746 W is applied and the power-factor field effectively carries the combined power factor and efficiency of the motor — a simplification, which is why motor circuits get an additional verification warning and should always be checked against NEC Article 430 by a licensed electrician.
Parallel Conductor Sets
Large feeders often run multiple conductors per phase in parallel. Two identical parallel sets halve the effective impedance, cutting voltage drop in half; three sets cut it to a third. The NEC permits parallel conductors 1/0 AWG and larger (with matching length, material, and size per NEC 310.10), so treat parallel results for small conductor sizes as theoretical.
This guide is provided for educational and reference purposes. Always confirm circuit design against the current NEC edition, local amendments, and the authority having jurisdiction before any electrical work.